With the T18 set bonuses,
Haste becomes very slightly better than Mastery. The reason is that Haste increases the number of
heroes you summon with the T18 2-piece bonus
(you cast spells faster, so you get more procs of Arcane Missiles) and increases
their damage (because they cast faster). In turn, the increased number of heroes
means increased uptime on the damage buff they give you through the
T18 4-piece bonus.

These statistics have been obtained by combining common sense, in-game testing,
and simulations using Simulation Craft.